Ceramics · Cone 10 / 1280°C
Eryri mineral glazes and reduction stoneware
Lowri’s stoneware glazes are built from what the valley gives: slate dust from the cutting bench, ash from the workshop stove and iron from the river bed.
Why we fire to Cone 10
At Orton cone 10, around 1280°C, a stoneware body vitrifies and the glaze melts into a continuous glass that locks onto the clay beneath. The result is a cup that does not weep and a plate that survives the dishwasher. It is also a long, hot climb that rewards patience and punishes shortcuts.
We fire in reduction. By restricting air to the burners we make the flame hunt for oxygen, and it takes some from the glaze and the clay. Iron gives it up readily: red-brown ferric oxide becomes ferrous oxide, a strong flux that tints silicate glass blue-green. That single chemical shift is the difference between a brown pot and a celadon one.
Slate dust from Bryn’s bench
Slate is mostly quartz, mica and chlorite with a little iron, so a handful of fine dust brings silica, alumina, potash and iron together. Lowri first suggested sieving the dust from Bryn’s cutting bench, and it has been in the glaze room ever since. We pass it through an 80-mesh sieve and add it in five per cent steps to test glazes and slips. It softens colour and gives a quiet, dark speckle.
Dust is a real hazard in any ceramic workshop. Respirable crystalline silica is the one to fear, so we sieve damp, never sweep dry, and wear a fitted respirator.
Ash from the hearth
Wood ash is mostly calcium carbonate with potash and a little silica and phosphate, which makes it a natural flux. Raw ash is caustic, so we wash it several times in a bucket, wearing gloves, decanting the cloudy water each time. Then we dry it, sieve it and test it. Ash glazes in the literature run from about twenty to fifty per cent of the recipe, so that is where our line blends begin.
Iron from the river bed
Ochre collects in the slow bends of Gwynedd streams. We settle it in buckets, pour off the water, dry the sludge, grind it and sieve it. Iron content differs between streams and between seasons, so each batch gets its own labelled test tile. A jar of river iron is never quite the same twice, and neither are the glazes it makes.
| Series | Iron oxide, % of dry glaze | What we look for |
|---|---|---|
| Pale celadon | 1.0–1.5 | Watery blue-green, pooling in carved lines |
| Deep celadon | 2.0–2.5 | Olive to sea-green, glassy depth |
| Tenmoku | 8–10 | Near-black, with rust breaks on the rims |
| Rust-break tenmoku | 10–12 | Persimmon red where the glaze runs thin |
Treat the grid as a starting point. Every kiln, clay and iron source behaves differently, and the tile always has the last word.
Test tiles and the firing
Every new batch gets a ladder of tiles, pressed with a groove so the glaze can pool and thin. We bisque to about 980°C, glaze, and fire the gas downdraught kiln over ten to twelve hours. Reduction starts at around 950°C and holds through the top end. Then we cool slowly and wait. Opening a kiln too early is the cheapest way we know to ruin a good firing.
Before anything leaves the workshop as tableware, we test sample pieces for lead and cadmium release. A glaze that looks like celadon but leaches is not celadon, it is a mistake.
To commission a table set, see the commissioning guide. Lead times are set by the kiln, which fires about once a month.